湿位涡和倾斜涡度发展

MOIST POTENTIAL VORTICITY AND SLANTWISE VORTICITY DEVELOPMENT

从完整的原始方程出发，在导出精确形式的湿位涡方程的基础上，证得绝热无摩擦的饱和湿空气具有湿位涡守恒的特性。并由此去研究湿斜压过程中涡旋垂直涡度的发展。结果表明，在湿等熵坐标中，涡旋的发展与对流稳定度的减少，等熵面上的辐合和潜热的释放有关。由于等熵位涡分析的应用受等熵面倾斜的限制，又进而发展了Ｚ坐标及Ｐ坐标中的倾斜涡度发展理论。指出无论是湿对称不稳定或对流不稳定大气，还是湿对称稳定或对流稳定大气，除对流稳定度的影响外，风的垂直切变的增加或水平湿斜压的增加均能因湿等熵面的倾斜而引起垂直涡度的增长。湿等熵面的倾斜越大，这种由干湿斜压性加强所引起的涡旋发展更激烈。在梅雨锋附近及其南侧暖湿区的北端，湿等熵面十分陡立，是涡旋发展及暴雨发生的重要地区。对１９９１年６月１２—１５日江淮流域暴雨过程的湿位涡分析表明，湿位涡分析，尤其是等压面上湿位涡量ＭＰＶ１和ＭＰＶ２的分析不仅在中高纬有效，在低纬度及低对流层也十分有效，是暴雨诊断和预报的有力工具。

An accurate form of the moist potential vorticity (MPV) equation was deduced from a complete set of primitive equations system. It was shown that motion in a saturated atmosphere without diabatic heating and frictional dissipation conserves moist potential vorticity.This property was then used to investigate the development of vertical vorticity in moist baroclinic processes. Results show that in the frame work of moist isentropic coordinate, vorticity development can result from reduction of convective stability, or convergence. or latent heat release in isentropic surfaces. However, the application of the usual analysis of moist isentropic potential vorticity is limitted due to the declination of moist isentropic surfaces, and a theory of slantwise vorticity development based on Z-coordinate and P-coordinate was then proposed. According to this theory, whether the atmosphere is moist symetrically stable or unstable. or convectively stable or unstable. so long as the moist insentropic surface is slantwise the reduction of convective stability, the increase of the vertical shear of horizontal wind or moist baroclinity can result in the increase of vertical vorticity. The larger the declination of the moist isentropes, the more vigorous the deveolopment of vertical vorticity. In a region with a monsoonal front to the north and warm and moist air to the south, or bye the north of front the moist isentropes are very stiff. This is the region most favorable for the development of vorticities and formation of torrential rain.For a case of persistent torrential rain occuriny in the middle and lower reaches of the Yangtze and Huai River in June 12-15, 1991. moist potential vorticity analysis, especially the isobaric analysis of its vertical and horizontal compoments, i e. MPVI and MPV2 respectively, is effective for indentifying synoptic systems not only in middle and high latitudes. but also in the low latitudes and in the lower troposphere. It can serve as a powerful tool for the diagnosis and prediction of torrential rain.